sys/unix/*.c formatting and PANICTRACE fix

I started out just reformatting the function header for regularize()
but ended up doing miscellaneous other stuff, including some code
changes.  I think the CHDIR code is a bit cleaner now, but shouldn't
have any differences in behavior.

Along the way I noticed that 'nethack -dpath' or 'nethack -d path'
modifies argv[] before PANICTRACE attempted to save argv[0], so would
break having nethack invoke gdb to get a backtrace.  ('ARGV0' seems to
be unnecessary since 'hname' holds the same value, but I didn't get rid
of it....)

include/decl.h

@@ -421,7 +421,7 @@ struct plinemsg_type {
 E struct plinemsg_type *plinemsg_types;
 
 #ifdef PANICTRACE
-E char *ARGV0;
+E const char *ARGV0;
 #endif
 
 #undef E